title: How strong is hydrogen bonding in ionic liquids? Combined X-ray crystallographic, infrared/Raman spectroscopic, and density functional theory study.
abstract: Hydrogen bonding in ionic liquids based on the 1-(2'-hydroxylethyl)-3-methylimidazolium cation ([C₂OHmim](+)) and various anions ([A](-)) of differing H-bond acceptor strength, viz. hexafluorophosphate [PF6](-), tetrafluoroborate [BF₄](-), bis(trifluoromethanesulfonimide) [Tf₂N](-), trifluoromethylsulfonate [OTf](-), and trifluoroacetate [TFA](-), was studied by a range of spectroscopic and computational techniques and, in the case of [C₂OHmim][PF6], by single crystal X-ray diffraction. The first quantitative estimates of the energy (E(HB)) and the enthalpy (-ΔH(HB)) of H-bonds in bulk ILs were obtained from a theoretical analysis of the solid-state electron-density map of crystalline [C₂OHmim][PF6] and an analysis of the IR spectra in crystal and liquid samples. E(HB) for OH···[PF6](-) H-bonds amounts to ~3.4-3.8 kcal·mol(-1), whereas weaker H-bonds (2.8-3.1 kcal·mol(-1)) are formed between aromatic C2H group of imidazolium ring and the [PF6](-) anion. The enthalpy of the OH···[A](-) H-bonds follows the order: [PF6] (2.4 kcal·mol(-1)) < [BF₄] (3.3 kcal·mol(-1)) < [Tf₂N] (3.4 kcal·mol(-1)) < [OTf] (4.7 kcal·mol(-1)l) < [TFA] (6.2 kcal·mol(-1)). The formation of aggregates of self-associated [C₂OHmim](+) cations is present in liquid [C₂OHmim][PF6], [C₂OHmim][BF₄], and [C₂OHmim][Tf₂N], with the energy of the OH···OH H-bonds amounting to ~6 kcal·mol(-1). Multiple secondary interactions in the bulk ILs influence their structure, vibrational spectra, and H-bond strength. In particular, these interactions can blue-shift the stretching frequencies of the CH groups of the imidazolium ring in spite of red-shifting CH···[A](-) H-bonds. They also weaken the H-bonding in the IL relative to the isolated ion pairs, with these anticooperative effects amounting to ca. 50% of the E(HB) value.
authors.affiliation: A. E. Arbuzov Institute of Organic and Physical Chemistry of Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov str. 8, 420088 Kazan, Russia. skatsyuba@yahoo.com
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Katsyuba
authors.forename: Mikhail V
authors.initials: MV
authors.lastname: Vener
authors.forename: Elena E
authors.initials: EE
authors.lastname: Zvereva
authors.forename: Zhaofu
authors.initials: Z
authors.lastname: Fei
authors.forename: Rosario
authors.initials: R
authors.lastname: Scopelliti
authors.forename: Gabor
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authors.lastname: Laurenczy
authors.forename: Ning
authors.initials: N
authors.lastname: Yan
authors.forename: Emilia
authors.initials: E
authors.lastname: Paunescu
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Dyson